Forward the Cache-Control header from frontends to backend caches

Implicit updates and only-if-cached requests depend on it, but currently
result in regular GETs on the backend. This results in broken refreshing and
unnecessary parsing of only-if-cached requests.

Change-Id: I0767f640518945ab315f29656b77dd08ce8d21a9
Merge "Forward the Cache-Control header from frontends to backend caches" into production